1. Field of the Invention
The present invention relates to optical disk units adapted for the optical writing and/or reading of high density information and more particularly to improvements in unit constructional features that protect record portions of the disk units, e.g. from dust or damage.
2. Description of the Prior Art
Optical disk units generally include a record stratum containing (or adapted to be recorded with) minute information marks (e.g. pits). Such small marks are extremely susceptible to damage, e.g. alteration, by contact and can be obscured by dust particles. Therefore, many optical disk unit constructions provide protection for the record stratum. For example, U.S. Pat. No. 4,038,663 and International Application WO 81/03712 disclose optical disk units wherein a transparent cover disk is centrally attached to the record element and, during operative rotation, flies above the record stratum. In both of these systems a stream of filtered air is introduced at a central position between the record stratum and cover sheet and flows outwardly due to centrifugal force imparted by the rotating disk unit.
U.S. Pat. Nos. 4,074,282 and 4,365,258 disclose disk units wherein a cover sheet is supported around its outer periphery in spaced relation to a record element so as to provide a substantially enclosed air space between the record element and the cover sheet. In the '258 patent the cover sheet is a flexible web material and cylindrically symmetric web tensioning assists in maintaining the cover sheet/record element spacing. While the use of flexible web cover sheets has advantages from optical and fabrication viewpoints, such webs are more susceptible to contacting the record surface, e.g. due to dimensional changes (causing sag) or due to flexure during handling. It is desirable to avoid contact between the cover sheet and record element and this militates toward disk unit designs having a greater spacing of the cover sheet from the record surface. However, a contrary design constraint exists because it is desirable to use small, lightweight write/read lenses of high numerical aperture and such lenses are necessarily located close to the record element surface. Thus, large spacing of a cover sheet from the record surface increases the risk of contact between the cover sheet and such lightweight, high numerical lenses during write/read operations.